As soon as children can sit unaided, shooting outdoors is a great option.
There is no posing and I don't ask them to sit still and smile nicely....we make it fun!
Child portraiture shoots have seen me fairy spotting, troll hunting, den making, flower picking, stick throwing, tree climbing, leaf collecting and dancing. I also get the low-down on favourite animals, the nicest teachers, the coolest toys, the best games.....you get the idea!! :-)
This approach makes the experience fun and engaging for the little ones, and as a result we get extremely candid photos.